Impact sensor

ABSTRACT

A magnetic impact sensor for motor vehicles with a safety system such as an airbag or belt tensioner for an occupant restraint system, has a circuit breaker arranged in an electrical trigger circuit of the safety system for inflating the airbag or tightening the belt by closing the trigger circuit in response to an acceleration or deceleration effective beyond a prescribed time duration. For this purpose a magnet in combination with specially shaped pole pieces forms two magnetic circuits the magnetic conductances of which are influenced by the position of a ferromagnetic ball that moves in response to an impact relative to the pole pieces to thereby open or close the circuit breaker. Normally, in the absence of an impact the ball is in a first position that keeps the circuit breaker open. When an impact occurs the ball moves into a second position to close the circuit breaker and thus the trigger circuit.

FIELD OF THE INVENTION

The invention relates to an impact sensor with a magnet which isespecially usable for safety devices such as an airbag in motorvehicles.

BACKGROUND INFORMATION

An acceleration sensor is, for example, described in the German PatentPublication DE 2,158,800. When an acceleration force arises, a sphericalball which in its resting position is attracted by a permanent magnet,is moved away from the magnet by overcoming the attraction force foractivating a switching operation. It is disadvantageous that the switchis a microswitch with an actuating push rod loaded by a spring force. Inorder to overcome the disadvantages of using springs in such systems, ithas also already been recommended in German Patent PublicationDE-3,338,287 to use a permanent magnet system in an acceleration sensorwhich comprises two permanent magnets facing each other with the samesign poles and which are slideably arranged in the direction of theirlengthwise axes. The permanent magnets are ring magnets within which areed switch is arranged. With such an arrangement perhaps one can avoidusing springs. However, operating conditions for the reed switch aremost difficult in practice; especially bouncing of the contacts isfrequent.

SUMMARY OF THE INVENTION

It is the object of the invention to provide a magnetic impact sensorfor safety devices, which functions exactly according to a structurallygiven characteristic, even under difficult application and switchingconditions and which achieves its proper function over a long workinglife.

This object has been achieved by an impact sensor according to theinvention, wherein the impact sensor has at least one magnet which formswith its respectively shaped pole pieces two magnetic circuitsmagnetically connected in parallel with each other. The magnet is, forexample, a permanent magnet. The magnetic pole pieces form a housing inwhich a ferromagnetic ball can move in response to impact forces torespectively influence one or the other magnetic circuit to thereby openor close a reed contact of a reed relay switch which is, for example,connected in a trigger circuit for activating a passenger safety devicein a vehicle. The ball moves perpendicularly to the longitudinalorientation of the reed contacts, whereby the ball, in each of its endpositions positively influences the respective magnetic circuit eitherto short circuit one of the two parallel magnetic circuits or topositively close the reed contacts

BRIEF DESCRIPTION OF THE DRAWINGS

An example embodiment of the invention is purely schematically

FIG. 1 shows two parallel magnetic circuits formed according to theinvention;

FIG. 1a shows an electric reed switch arranged to be operated by themagnetic circuits of FIG. 1;

FIG. 1b is a lengthwise section through a sensor housing enclosing afree space in which a mass in the form of a spherical ball is movable inresponse to acceleration;

FIG. 1c is a top view onto another free space, wherein the sphericalball is free to move in response to acceleration;

FIG. 1d shows a modification of the present sensor with a funnel shapedhousing for enclosing a free space in which the spherical ball is freeto move in response to acceleration, whereby the free space is shapedfor using gravitational force for resetting the ball in addition to themagnetic force and for a characteristic substantially determined by thefunnel shape; and

FIG. 1e is a sectional view along section line 1e--1e

DETAILED DESCRIPTION OF PREFERRED EXAMPLE EMBODIMENTS AND OF THE BESTMODE OF THE INVENTION

Referring to FIG. 1 the present impact sensor comprises a permanentmagnet 1 and two soft magnetic circuits 1a and 1b normally connected inparallel with each other and having different magnetic resistances orconductances 8 and 9. The magnetic circuit with the lower magneticresistance 8 is normally closed by a spherical ferromagnetic ball 2which normally stays in a first position when there is no impact. Theball 2 is subject to gravity and also can move in response to impactforces. Under the effects of an acceleration or deceleration caused byimpact forces over a defined time, the ball 2 moves out of its initialfirst position, whereby the magnetic resistance increases sharply. Thus,the magnetic potential rises and a reed relay 5 shown in FIG. 1a locatedin the circuit having the higher magnetic resistance 9 and acting as amagnetically activated turn-on switch, closes its reed contacts 5aconnected at 12, 12a to a trigger circuit for turning on a safety devicenot shown. If the effect of the external impact forces stops, then thespherical ball 2 is again pulled back or reset into the initial positionby the magnetic field of the magnet 1. This resetting may be supportedby a reset spring not shown. The reed delay 5 again assumes the originalswitching position shown in FIG. 1b in which the reed contacts 5a areopen as shown in FIG. 1a. The reed relay 5 has its contacts 5a inside aglass jacket 5b.

FIG. 1b shows the construction of the magnetic impact sensor of theinvention. The response characteristic may be influenced by shaping thefree space 10 for the ball 2 in the x-y-plane and in the Z-direction.Shaping in the x-y-plane involves, for example, a circular segment 10ashown in FIG. 1c. Shaping is also possible or necessary in thez-direction by an elliptical segment. The response threshold, theresponse sensitivity, and the switch-on duration must be substantiallyadapted to the desired values by means of the structural details. A highreliability, for example, in the trigger circuit of an airbag system, abelt tensioner or similar known safety systems, is achieved with thereed relay 5 which assures the electrical contact closing.

Referring further to FIG. 1b the magnet 1 has a north pole N in magneticcontact with a magnetic pole piece 3, and a south pole S in contact witha pole piece 4. The pole pieces 3 and 4 are held together by anelectrically and magnetically non-conducting element 7. The abovementioned magnetic circuit 1a is formed by the pole piece 3, a slantedpole piece element 3a, through the ball 2 in its shown normal position,through a slanted pole piece element 4a, and the pole piece 4, back tothe magnet 1. In the shown normal position the ball 2 formssubstantially a magnetic short circuit, which is shown symbolically inFIG. 1 as a closed switch representing the magnetic conductance 8. Thesecond magnetic circuit includes the ferromagnetic pole piece elements3, 3a, 3b, 3c, the ball 2, and ferromagnetic pole piece elements 4c, 4b,4a, 4. The ball 2 is closing the second magnetic circuit only inresponse to an impact force to influence the magnetic conductance 9. Theelements 3b and 4b are held together by an electrically and magneticallynonconducting housing section 6 which encloses an inner chamber formingthe above mentioned free space 10 for the ferromagnetic ball 2, forexample, in the shape of a spherical ball. The above mentioned polepiece elements 3, 3a, 3b, 3c, and 4c, 4b, 4a, 4 form part of a housingand are all made of ferromagnetically conducting sheet metal.

The reed switch 5 is supported in the electrically and magneticallynon-conducting section 6 between pole piece elements 3c and 4c formingmagnetic yoke plates.

Referring to FIG. 1c, the characteristic curve of the magnetic forceeffect on the ball 2, is substantially freely influenced by the shape ofthe free space in the housing in which the ball 2 can move. The shape ofthe inner housing walls in FIG. 1c is substantially semi-circular toenclose a respective free space 10a which also determines the boundariesof the motion of the ball 2 out of its normal first or rest position 2'between the pole piece extensions 3a and 4a close to the magnet 1, intothe switching position 2" close to the reed relay 5 with reed contacts5a. In the latter position, the ball 2 is located temporarily betweenthe pole piece elements 3c and 4c, forming magnetic yoke plates in thesecond magnetic circuit 1b.

The magnetic yoke plates 3c and 4c in FIG. 1b are arranged behind oneanother to the extent to which they overlap each other, whereby the yokeplate 4c partly covers the yoke plate 3c. FIG. 1e shows the arrangementsof the respective elements in FIG. 1b sectioned along section line1e--1e.

FIG. 1d shows a modification in which a magnet 13 having a north pole Nand a south pole S, as in FIG. 1b, is located next to a resting seat 11formed at a transition between a conical housing section 14 and atubular housing section 14a. The same ferromagnetic ball 2 as in theother Figures is normally held in the resting seat 11 by the force ofthe magnet 13. However, in FIG. 1d the magnet 13 is arranged in thetubular housing section between the resting seat 11 for the ball 2 andthe reed relay 5. When the ball 2 is in the seat 11 due to the magneticattractive force and due to the force of gravity the contacts 5a of thereed relay 5 are open as shown. When the ball 2 is forced out of theseat 11 by an impact force, the contacts 5a close, because now themagnetic circuit through the reed contacts 5a offers a lower magneticresistance than is present when the magnetic field lines can passthrough the ball 2 in its resting position in the seat 11. When theimpact force ceases, the ball will automatically be restored in itsresting position and the contacts 5a open again.

Although the invention has been described with reference to specificexample embodiments, it will be appreciated, that it is intended tocover all modifications and equivalents within the scope of the appendedclaims.

We claim:
 1. A magnetic impact sensor, comprising a sensor housing (6,7)a reed relay mounted in said sensor housing and having at least two reedcontacts, magnet means (1) for operating said reed contacts, said magnetmean shaving a north pole (N) and a south pole (S), means operativelymounting said magnet means in said sensor housing ferromagnetic polepiece means (3, 3a, 3b, 3c; 4, 4a, 4b, 4c) for forming two magneticcircuits arranged in parallel to each other, a ferromagnetic mass (2)movable in response to an impact force in a free space (10) in saidsensor housing, said ferromagnetic pole piece means forming said firstmagnetic circuit through a first magnetic path (N, 3, 3a; 4a, 4, S) andthrough said ferromagnetic mass (2) when the latter is in a restposition in said first magnetic path while an impact force is absent,said ferromagnetic pole piece means also forming said second magneticcircuit through a second magnetic path (N, 3, 3a, 3b, 3c; 4c, 4b, 4a, 4,S) and through said ferromagnetic mass (2) when the latter is in adisplaced position in said second magnetic path while an impact force iseffective, said at least two reed contacts being normally open andextending substantially perpendicularly to a movement direction of saidferromagnetic mass (2), said reed relay being positioned in said secondmagnetic path, so that said normally open reed contacts are closed whensaid impact force is effective and open when said ferromagnetic mass isin said rest position while an impact force is absent.
 2. The magneticimpact sensor of claim 1, wherein said free space in said housing meanshas an approximately semicircular horizontal cross-section.
 3. Themagnetic impact sensor of claim 1, wherein said pole piece meanscomprise two slanted sections (3a, 4a) forming a seat for saidferromagnetic mass (2) in its rest position, and two spaced yoke plates(3c, 4c) forming a gap in which said ferromagnetic mass is received whensaid mass is displaced by an impact force.
 4. A magnetic impact sensor,comprising a magnet (13) having a north pole (N) and a south pole (S)arranged in a common plane, a housing having a conical housing section(14) and a tubular housing section forming a seat (11) where saidconical housing section merges into said tubular housing section, aferromagnetic mass (2) normally resting on said seat in the absence ofan impact force, said magnet (13) being mounted in said tubular housingsection, so that said common plane of said north and south poles extendsin parallel to a plane defined by said seat (11), a reed relay (5) alsomounted in said tubular housing section, so that reed contacts of saidreed relay extend substantially in parallel to said common plane of saidnorth and south poles, whereby said reed contacts of said reed relay arenormally held open as long as said ferromagnetic mass (2) rests on saidseat, and wherein said reed contacts are closed when said ferromagneticmass (2) is removed from said seat in response to an impact force. 5.The magnetic impact sensor of claim 4, wherein said conical housingsection (14) encloses a free space in which said ferromagnetic mass (2)is movable in response to an impact force.
 6. The magnetic impact sensorof claim 4, wherein said magnet (13) is located in said tubular housingsection between said seat (11) and said reed relay.